Problem 54 - Chemistry The Molecular Science

Question

The surface tension of a liquid decreases with increasing temperature. Using the idea of intermolecular attractions, explain why this is so.

Step-by-Step Solution

Answer

Surface tension decreases with temperature because increased molecular motion weakens intermolecular attractions.

Step 1: Understand Surface Tension

Surface tension is a property of a liquid's surface that makes it behave like an elastic sheet. It results from the cohesive forces between liquid molecules.

Step 2: Identify Key Determinants of Surface Tension

Intermolecular attractions are the main determinants of surface tension. Strong intermolecular forces result in higher surface tension as molecules are more tightly held together.

Step 3: Analyze the Effect of Temperature on Molecules

Increasing temperature provides energy to the molecules, causing them to move more quickly and increase in kinetic energy. This movement weakens the attractive forces between the molecules.

Step 4: Relate Temperature and Surface Tension

As temperature increases and intermolecular attractions weaken, molecules at the surface can spread apart more easily, decreasing the surface tension.

Key Concepts

Intermolecular Attractions

Intermolecular attractions are the forces that hold molecules together within a liquid. These attractions include van der Waals forces, hydrogen bonds, and dipole-dipole interaction. Each of these forces plays a critical role in determining how tightly molecules are held together.

- **Van der Waals Forces**: These are weak attractions that occur between all molecules. They arise from temporary dipoles that occur when electrons within a molecule temporarily crowd at one part, creating an attraction between molecules. - **Hydrogen Bonds**: Stronger than van der Waals forces, hydrogen bonds occur when a hydrogen atom is covalently bonded to a more electronegative atom like oxygen or nitrogen. This type of bond is particularly significant in water, giving it a high surface tension. - **Dipole-Dipole Interaction**: These are attractions between polar molecules where positive ends are attracted to negative ends of another molecule.
Surface tension relates directly to these forces—stronger attractions mean tighter, more bonded molecules at the surface. This is why liquids like water, which have strong hydrogen bonds, have high surface tension.

Temperature Effects on Liquids

Temperature has a profound effect on liquids by influencing the speed and movement of the molecules within. As the temperature of a liquid increases, the molecules gain kinetic energy, causing them to move more rapidly.

• Increased movement disturbs the orderly arrangement of molecules, allowing them to break away from the cohesive forces more easily.
• As molecules move faster, they collide more frequently and with more force. This agitation weakens the intermolecular forces that are holding them together.

The result of this increased molecular movement is a decrease in surface tension. The molecules at the surface have enough energy to stay less tightly packed, causing the surface tension to drop. This phenomena is why you may notice that warm water spreads out more easily than cold water.

Cohesive Forces

Cohesive forces are the attractive forces between molecules in a liquid. These forces are responsible for holding the liquid together and for the formation of the surface layer that acts like a stretched elastic sheet.

- **Surface Layer**: The molecules at the surface are pulled inward by cohesive forces, which minimize surface area. - **Cohesion in Practice**: In water, cohesion is primarily due to hydrogen bonding, which holds water molecules tightly together. - **Impact on Surface Tension**: Strong cohesive forces mean high surface tension, because molecules at the surface are more reluctant to escape into the air or spread out.
When the temperature increases, the kinetic energy of molecules overcomes these cohesive forces, causing a reduction in surface tension. This interplay between cohesive forces and temperature is crucial in understanding why warm liquids behave differently in terms of surface interaction compared to cold liquids.